1. Field of the Invention
The invention relates to a filtration system for filtering a molten plastic flow. More specifically, the invention relates to such a filtration system which includes a rotatable screening disc having an annularly arranged screening zone comprising a plurality of cavities separated by spacing webs, the system being modified to operate with an increased active filter area.
2. Description of Prior Art
As is known in the prior art, in any filtration system, the filtering screens must, from time to time, be changed to cleanse them of the impurities they have captured in operation. Filtration systems with screen changing arrangements are taught in, for example, U.S. Pat. No. 4,167,384, Shirato et al, Sept. 11, 1979, U.S. Pat. No. 4,395,212, Lambertus, July 26, 1983, U.S. Pat. No. 4,588,502, Zibell et al, May 13, 1986, German Patentschrift DE No. 3 302 343 C2, Rehau Plastics AG, Oct. 24, 1985, German Offenlegungsschrift DE No. 33 02 343 A1, Gneuss, Aug. 2, 1984, German Patentschrift No. 33 02 343 C2, Rehau Plastics AG, Oct. 24, 1985, German Patentschrift DE No. 33 41 508 C1, Gneuss, Dec. 13, 1984. The Shirato et al patent uses two parallel screens, while the Lambertus patent teaches the use of a sliding plate with several screens. Neither of these relate to a system using a screening disc having a plurality of screens arranged in an annular screening zone therearound, as does the present invention, and are therefore not relevant having regards to the teachings of the present disclosure.
The remainder of the patents teach a system having such an annular screening disc. Thus, the '343 A1 and '343 C2 patents teach such a system. In both of these patents, the circular disc is housed in a housing which includes plates on either side of the disc, each of the plates having aligned flow-through bores. The disc is mounted in the housing such that there is always at least one cavity aligned with the aligned flow-through bores. The cavity will include a screen stack to filter the molten plastic of the molten plastic flow. In both of the German applications, the flow-through bores are circular in cross-section. Thus, the flow channel through the filtration system, consisting of the aligned flow-through bores and an aligned cavity, are virtually circular in cross-section.
The '502 patent teaches a similar system to the systems taught in the German references above-referred to. However, this patent teaches the added improvement of providing evacuation of a cavity preceding the cavity in the flow-through channel.
There are disadvantages to the system as described in the two German references and the '502 U.S. patent. Thus, the active filter area of this system is limited to approximately the cross-sectional area of the molten plastic flow in the round flow channel entering the filter. If it is attempted to increase the active filter area by simply increasing the circular area of the bores, then the material in the outer periphery of the increased area would tend to move so slowly that this material could be thermally deteriorated.
It is well known that an increase in the size of the active filter area is desirable so that a system which provides facilities for an increased active filter area has advantages over the systems as taught in the three above references.
The '502 U.S. patent notes that, when a new cavity enters into the flow channel, the air in the cavity entering the flow channel has to be removed. The '502 patent proposes that the cavity in advance of the cavity in the flow-through channel be evacuated so that when the evacuated cavity is subsequently inserted into the flow-through channel, it will not provoke any disturbances in the molten plastic nor will air from the channel be caught in the molten plastic flow.
However, with the system of the '502 patent, the cavity, on entering the flow-through channel, does not commence to be flooded until such time as the vacuum has already been turned off. Material in the bore-through holes of the cavity can, under these conditions, and due to thermal effects on the material in the bore-through holes, build up gases which will enter the cavity to break the vacuum to thereby erase the advantages of evacuating the cavity. It is pointed out that the vacuum will be off for perhaps as much as an hour until a cavity, on entering the flow-through channel, starts being flooded. It would therefore be of an advantage to provide a system wherein the cavity which is being flooded is still connected to the vacuum source at the same time that the cavity is being flooded.
Finally, in the above-described systems, when a new cavity enters the flow-through channel, there is a pressure drop in the system. This causes a pulsating type of response in the system which is unsatisfactory.
The "pulse" will increase with the volume of each cavity. Accordingly, there is an advantage to providing smaller sized (i.e., more) cavities.
In addition, an increased active filter area combined with smaller cavities increases the through-put capabilities of the filtration system while providing a steadier flow.